The oil and gas industry represent a tremendous global energy growth sector, with over 5e5 active well-pads in the United States (US) alone. Natural gas (NG) has received particular interest, given its potential as an intermediate "bridge fuel" prior to full migration into renewable energy sources; however, the primary constituent of NG is methane (CH4) which presents a radiative forcing > 20x higher than CO2. Given the significant expansion of oil and gas drilling operations in the United States, the efficacy of NG as a clean fuel is dependent upon minimization of fugitive emissions during NG extraction, processing, and delivery, which requires source magnitude estimation and localization capability for leak detection and repair (LDAR). The principal challenge therefore involves the development of high-fidelity source estimation technologies for spatially and temporally resolved CH4 emissions monitoring, to enable fast and site-specific LDAR competency.

We present a new fringe mitigation method based on Fourier decomposition of background optical fringe patterns. Application to a silicon photonic on-chip sensor show order of magnitude improved concentration retrieval stability.

Recent advances in hybrid integrated silicon photonic (SiPh) technologies are enabling the migration of conventional free-space optical spectroscopic sensors onto a compact on-chip platform [1-3]. In addition to the small spatial footprint and power efficiency, we envision such sensors to be scalably manufactured using existing CMOS-compatible foundry processes, thus providing disruptive SWaP-C (size, weight, power, and cost) benefits in contrast to commercially available optical sensors. Initial demonstration of evanescent TDLAS (tunable diode laser absorption spectroscopy) of methane (CH4) on a passive SiPh waveguide has indicated minimum fractional absorption of 3.3e-5 Hz-1/2, which is on-par with state-of-art open-path TDLAS sensor systems [4]. Given the general recent movement toward cleaner fuels, CH4 fugitive emissions monitoring is of significant interest given the extremely high radiative forcing potential [5]. For a nominal waveguide length of 30 cm with 25 % evanescent exposure, this corresponds to ~ 10 ppmv detection sensitivity at 1 s integration time, and further sensitivity enhancement is expected with even longer waveguides, as the laser RIN typically dominates our measurements at nominal waveguide lengths. Despite the excellent sensitivities for short-term integration periods, long-term measurements (> 10 s) are potentially limited on a silicon platform due to the high material thermo-optic coefficient, resulting in significant susceptibility of Fabry-Perot etalons to drift in the presence of even small (~ 1 mK) thermal fluctuations. To this end, customized spectral fitting algorithms have played a significant role in both fringe drift mitigation and peak detection fidelity (e.g. in the presence of a passing CH4 plume), which are crucial for enhancing long-term stability without the need for frequent sensor recalibration. A variety of spectral algorithms have been designed for this purpose, and details will be presented at the meeting.

We present comprehensive characterization of silicon photonic sensors for methane leak detection. Sensitivity of 40 ppmv after 1 second integration is reported. Fourier domain characterization of on-chip etalon drifts is used for further sensor improvement.

Radical species play an important role in various chemical processes spanning atmospheric chemistry (eg ozone formation), bio-medical science, and combustion. These highly reactive chemicals usually occur at very low concentration levels, and are difficult to quantify in experiments. Generally, laser-based techniques rely on careful selection of the target transition to minimize spectral interference and achieve high selectivity. In case of complex gas mixtures (such as air) a possibility of spectral interference always exists. Since Faraday rotation spectroscopy (FRS) is sensitive only to paramagnetic species (radicals), it can simultaneously provide ultra-high sensitivity and selectivity.

We have studied various spectroscopic detection techniques for applications in multi-heterodyne spectroscopy performed in a dual comb configuration. Self-coherence properties of a multi-mode QCL are also investigated.

In this dissertation, laser spectroscopy is utilized to monitor trace-gas species for environmental and health applications. Due to their non-invasive and in situ sensing capabilities, optical platforms are attractive for on-site, real-time diagnostics. Two main techniques are investigated: (i) Faraday rotation spectroscopy (FRS) and (ii) tunable diode laser spectroscopy (TDLS), where noise reduction techniques are implemented in both cases for precise and accurate quantification of analytes. A variety of sensing configurations are demonstrated, including benchtop laboratory sensors [Chapters 4, 6, 7], transportable extractive point sensors [Chapter 5], and on-chip integrated sensors [Chapter 7].

Multiheterodyne spectroscopy implemented with semiconductor Fabry-Perot lasers is a method for broadband (> 20 cm-1), high spectral resolution (~1 MHz) and high time resolution (< 1 us/spectrum) spectroscopy with no moving parts utilizing off-the-shelf laser sources. The laser stabilization approach demonstrated here enables continuous frequency tuning (at 12.5 Hz repetition rate) while allowing for multiheterodyne wavelength modulation spectroscopy (WMS). Spectroscopic detection of N2O around 1185 cm-1 is experimentally realized, which shows a direct absorption sensitivity limit of ~1.5*10-3/rt(Hz) fractional absorption per mode. This can be lowered using WMS down to 5*10-4/rt(Hz) per mode, limited by optical fringes. This approaches the range of sensitivities of standard single-mode laser based spectrometers, which demonstrates that the multiheterodyne method is well-suited for chemical sensing of spectrally broadened absorption features or for multi-species measurements.

Spectroscopic chemical sensing based on semiconductor lasers is already an established method for highly sensitive and selective detection. However, most semiconductor lasers have a rather narrow tuning range (< 1 %) which limits the detectable molecular species. A common solution to this problem is to use external cavity lasers (tuning range > 10 %) which allow for broadband and multi-species detection at the cost of reduced tuning speed, decrease in robustness, and generally higher cost. A particularly elegant way to perform a broadband and high resolution spectroscopic detection is the dual-comb spectroscopy technique [1]. This field has recently received some attention due to the possibility of replacing the mode-locked frequency combs with multi-mode Fabry-Perot (FP) semiconductor lasers to enable robust and cost efficient design.

Measurement of NO and/or its metabolites in the various body compartments has transformed our understanding of biology. The inability of the current NO measurement methods to account for naturally occurring and experimental NO isotopes, however, has prevented the scientific community from fully understating NO metabolism in vivo. Here we present a mid-IR Faraday rotation spectrometer (FRS) for detection of NO isotopes. The instrument utilizes a novel dual modulation/demodulation (DM) FRS method which exhibits noise performance at only 2 times the fundamental quantum shot-noise level and provides the record sensitivity in its class. This is achieved with a system that is fully autonomous, robust, transportable, and does not require cryogenic cooling. The DM-FRS enables continuous monitoring of nitric oxide isotopes with the detection limits of 3.72 ppbv.Hz-1/2 14NO and 0.53 ppbv.Hz-1/2 15NO using only 45 cm active optical path. This DM-FRS measurement method can be used to improve the performance of conventional FRS sensors targeting other radical species. The feasibility of the instrument to perform measurements relevant to studies of NO metabolism in humans is demonstrated.

Faraday rotation spectroscopy (FRS) of O2 is performed at atmospheric conditions using a DFB diode laser and permanent rare-earth magnets. Polarization rotation is detected with a hybrid-FRS detection method that combines the advantages of two conventional approaches: balanced optical-detection and conventional FRS with an optimized analyzer offset angle for maximum sensitivity enhancement. A measurement precision of 0.6 ppmv.Hz-1/2 for atmospheric O2 has been achieved. The theoretical model of hybrid detection is described, and the calculated detection limits are in excellent agreement with experimental values.

Faraday rotation spectroscopy is used for measurement of O 2 10 at atmospheric conditions. Low operating powers (< 10 W) are achieved by using rare-earth magnets for DC magnetic field generation. A sensitivity of 1.7 ppmv/Hz is demonstrated.

Faraday rotation spectroscopy is used for measurement of O2 at atmospheric conditions. Low operating powers (< 10 W) are achieved by using rare-earth magnets for DC magnetic field generation. A sensitivity of 1.7 ppmv/rt(Hz) is demonstrated.

We propose and show that for coupling modulated lasers (CMLs), in which the output coupler is modulated rather than the pump rate, the conventional relaxation resonance frequency limit to the laser modulation bandwidth can be circumvented. The modulation response is limited only by the coupler. Although CMLs are best suited to microcavities, as a proof-of-principle, a coupling-modulated erbium-doped fiber laser is modulated at 1 Gb/s, over 10000 times its relaxation resonance frequency.

An etch method based on surface tension driven flows of hydrofluoric acid microdroplets for the fabrication of low-loss, subwavelength-diameter biconical fiber tapers is presented. Tapers with losses less than 0.1 dB/mm are demonstrated, corresponding to an order of magnitude increase in the optical transmission over previous acid-etch techniques. The etch method produces adiabatic taper transitions with minimal surface corrugations. A biconical fiber taper fabricated using this method is used to demonstrate an erbium doped silica microsphere laser.